1. Field of the Invention
This invention relates to safety systems, and more particularly to those systems which require the closing of a normally open contact to put the system into a more permissive state.
2. Background Information
It is well known in the art, particularly in railroad safety systems and the like, to design the systems to be sensitive to conditions that would threaten the safety of passengers, and to impose requirements on components so as to insure that those components will fail in a manner that will guarantee the safety of the passengers. For example, it is well known in the railroad signalling art to utilize what are termed vital relays, such relays being constructed such that no failure can cause the front contacts to be closed if the coil is not energized.
Thus, safety systems generally require the closing of a normally-open contact to put the system into a more permissive state. If the normally-open contact is on a vital relay, it is directly accepted that if the relay is not energized, the contact is open. However, a difficulty is presented in that vital relays are relatively slow acting and their contacts may not be suitable for applications requiring very large numbers of operations in a given time period, or for interrupting or breaking large currents in circuits. In order to solve such difficulties, specialized relays have been developed such as mercury wetted reed relays; or in some cases, high current contactors may be required. However, this is not a satisfactory answer when safety is a prime consideration because the specialized relays are non-vital relays and they must satisfy the general criterion noted above for vital relays when they are used in safety systems. For example, there must be the ability to prove that a contact is operating as intended and there must be specified insulation resistance and breakdown voltage level to the relay frame, to other contacts etc.
Accordingly, certain multi-contact relays may be acceptable for use in vital circuits provided they satisfy the previously noted insulation and breakdown voltage criteria and if it is certain that all contacts operate together; that is, they are mechanically connected by an insulated pusher. Therefore, if a back contact on a relay of this type is closed, it may be accepted that no front contact is closed.
It will be appreciated however, that on certain other types of relays, for example on reed contact relays there is no mechanical connection between contacts. Thus, even though several reed contact capsules may be exposed to a common magnetic field, there is no assurance that all contacts will respond in the same way.
Accordingly, it is a primary object of the present invention to provide a means to prove that a contact is operating as intended.
Another object is to provide such means without compromising insulation and breakdown voltage rating of the relay.
Yet another object is to provide a checking circuit that will assure that a predetermined contact is operating properly, whether that be in a dynamic or static situation.